This invention relates to a dielectric barrier discharge lamp.
The majority of the known and commercially available low-pressure discharge lamps are so-called compact fluorescent lamps. These lamps have a gas fill which also contains small amount of mercury. Since mercury is a highly poisonous substance, novel types of lamps are being recently developed. One promising candidate to replace mercury-filled fluorescent lamps is the so-called dielectric barrier discharge lamp (shortly DBD lamp). Besides eliminating the mercury, it also offers the advantages of long lifetime and negligible warm-up time.
As explained in detail, for example, in U.S. Pat. No. 6,633,109 the operating principle of DBD lamps is based on a gas discharge in a noble gas (typically Xenon). The discharge is maintained through a pair of electrodes, between which there is at least one dielectric layer. An AC voltage of a few kV with a frequency in the kHz range is applied to the electrode pair. Often, multiple electrodes with a first polarity are associated to a single electrode having the opposite polarity. During the discharge, excimers (excited molecules) are generated in the gas, and electromagnetic UV and VUV radiation is emitted when the meta-stable excimers dissolve. The electromagnetic radiation of the excimers is converted into visible light by suitable luminescent material, in a physical process similar to that occurring in mercury-filled fluorescent lamps. This type of discharge is also referred to as dielectrically impeded discharge.
As mentioned above, DBD lamps must have at least one electrode set which is separated from the discharge gas by a dielectric. It is known to employ the wall of the discharge vessel itself as the dielectric. In this manner, a thin film dielectric layer may be avoided. This is advantageous because a thin film dielectric layer is complicated to manufacture and it is prone to deterioration. Various discharge vessel-electrode configurations have been proposed to satisfy this requirement. U.S. Pat. No. 5,994,849 discloses a planar configuration, where the wall of the discharge vessel acts as a dielectric. The electrodes with opposite polarities are positioned alternating to each other. The arrangement has the advantage that electrodes do not cover the discharge volume from at least one side, but a large proportion of the energy used to establish the electric field between the electrodes is dissipated outside the discharge vessel. On the other hand, a planar lamp configuration cannot be used in the majority of existing lamp sockets and lamp housings, which were designed for traditional incandescent bulbs.
U.S. Pat. Nos. 6,060,828 and 5,714,835 disclose substantially cylindrical DBD light sources, which are suitable for traditional screw-in sockets. These lamps have a single internal electrode within a discharge volume, which is surrounded on the external surface of a discharge vessel by several external electrodes. It has been found that such an electrode configuration does not provide a sufficiently homogenous light, because the discharge within the relatively large discharge volume tends to be uneven. Certain volume portions are practically completely devoid of an effective discharge, particularly those volume portions, which are further away from both electrodes.
U.S. Pat. No. 6,777,878 discloses DBD lamp configurations with elongated electrodes that are arranged on the inside of the wall of a cylindrical discharge vessel and are covered by a dielectric layer. In this configuration the electrodes are in a relatively large distance from each other, therefore a very high voltage is required to start ignition. In order to overcome cold starting difficulties an external metal ring is suggested at one end of the elongated cylindrical discharge vessel. This lamp configuration belongs to the group of DBD lamps of traditional elongated cylindrical shape and cannot be used as a replacement of an incandescent lamp.
Although DBD lamps are more environmentally friendly because of eliminating the need for mercury and also offer the advantage of long lifetime and negligible warm-up time, they still have one drawback, a relatively low efficiency. Several attempts have been made in order to overcome this disadvantage. U.S. Pat. No. 5,604,410 for example proposes generating a specific train of voltage pulses of a predetermined pulse form pulse time and idle time in order to increase the efficiency of BDB lamps. The improvement in efficiency achieved by this method of operation is however limited.
Another attempt is made by an invention disclosed in US continuation in part application of Ser. No. 11/112,320 filed by the present applicant on Apr. 22, 2005, in which electrodes with a dielectric layer acting as a cathode and an anode inside the discharge volume are used. By appropriate selection of the number and the geometry of arrangement of the electrodes, a substantial improvement of the output luminous efficiency can be accomplished. In these lamps, the inside surface of the discharge vessel may be covered with a luminescent layer containing phosphor. In such an arrangement, the surface area covered with the luminescent layer limits the output luminosity. As this surface area is determined by the lamp geometry, the output luminosity of such a lamp cannot be increased to an extent, as it would be desirable.
Accordingly, there is a need for a DBD lamp configuration with an improved efficiency and luminous output. It is sought to provide a DBD lamp, which, while having an improved discharge vessel-electrode arrangement, is relatively simple to manufacture. Further, it is sought to provide a discharge vessel-electrode configuration, which readily supports different types of electrode set configurations, according to the characteristics of the used discharge gas, exciting voltage, frequency and exciting signal shape. The proposed DBD lamp can be used as a replacement of the traditional incandescent lamps and fluorescent lamps containing mercury. It has an electrode arrangement, which minimizes the self-shadowing effect of the electrodes in order to provide for a higher luminance and efficiency.